1. Field of the Invention
The present invention relates generally to the drilling of large diameter shafts and, more particularly, to the drill bits utilized in such applications.
2. Description of the Prior Art
Large diameter bits have been used in mines for the drilling of rescue shafts, ventilation shafts and access shafts. Such bits have been designed in diameter ranges from approximately twenty-eight inches to one-hundred forty-four inches and larger.
Conventional large diameter bits comprise a flat bottom body forming a support structure for a plurality of rolling cutters. These cutters are frusto-conical in shape and are usually rotatively mounted on yokes which in turn are welded to the bit body base plate. Various sizes of cutters or combination of cutters can be mounted, either in the gage (periphery) positions or in the inner positions. The drill bits can be utilized as a raise drill or as a blind hole borer, drilling either in the upward or downward direction. The raise bit is used in a raise drilling operation to provide a relatively large diameter hole from a first mine level to a second mine level. The raise drilling operation begins by drilling a small diameter pilot hole through the earth from a first location to an opening at a second location using a small diameter pilot bit. After the pilot hole is completed, the pilot bit is removed from the drill column and the raise bit is attached thereto. The raise bit is rotated with an upward vertical pull and drawn along the pilot hole thereby enlarging the pilot hole to the desired size.
A major problem with large diameter bits is that they are quite difficult to transport through small drifts or passages and very cumbersome to maneuver inside such structures. Quite often the size of these passages severly restricts the type and size drill bit utilized.
Another problem encountered is that various size shafts are required for different applications in a single mine location. As a result either various sizes of drill bits must be available, which is quite costly, or compromises must be made in hole size selection.